[linux-lvm] Re: [RFLART] kdev_t in ioctls

[linux-lvm] Re: [RFLART] kdev_t in ioctls

[Linus Torvalds]

On Mon, 14 Jan 2002, Alexander Viro wrote:
>
> 	Linus, at least some ioctls (e.g. lvm ones) pass kdev_t from/to
> userland.  While the common policy with ioctls is "anything goes", this
> kind of abuse is IMNSHO over the top.

That's completely bogus.

The good news is that the bit-for-bit representation of old kdev_t and
"dev_t" are obviously 100% the same, so we should just make the damn thing
be dev_t, and user land will never notice anything.

So we can just change all structures that are exported to user land to use
"dev_t", and add the required conversion magic. Possibly by duplicating
the structure, and having "used_lvm_struct_x" and functions to read and
write them from/to user space.

> Public statement along the lines "any API that passes kdev_t values
> across the kernel boundary is unacceptable" would be a nice thing...

Consider that done. ANYTHING that exports kdev_t to user space is
incredibly broken, and will not work in a few months when the actual bit
representation (and size) will change.

Do we have any lvm people willing to fix this? (linux-lvm cc'd, but I know
they've been very silent on the 2.5.x changes so far)

		Linus

Re: [linux-lvm] Re: [RFLART] kdev_t in ioctls

[Christoph Hellwig]

On Mon, Jan 14, 2002 at 10:01:25AM -0800, Linus Torvalds wrote:
> 
> On Mon, 14 Jan 2002, Alexander Viro wrote:
> >
> > 	Linus, at least some ioctls (e.g. lvm ones) pass kdev_t from/to
> > userland.  While the common policy with ioctls is "anything goes", this
> > kind of abuse is IMNSHO over the top.
> 
> That's completely bogus.
> 
> The good news is that the bit-for-bit representation of old kdev_t and
> "dev_t" are obviously 100% the same, so we should just make the damn thing
> be dev_t, and user land will never notice anything.

Glibc disagrees with you (bits/types.h):

typedef __u_quad_t __dev_t;             /* Type of device numbers.  */

We'd have to use __kernel_dev_t instead which again pulls kernel
headers in..

	Christoph

Re: [linux-lvm] Re: [RFLART] kdev_t in ioctls

[Christoph Hellwig]

On Mon, Jan 14, 2002 at 07:08:34PM +0100, Christoph Hellwig wrote:
> > The good news is that the bit-for-bit representation of old kdev_t and
> > "dev_t" are obviously 100% the same, so we should just make the damn thing
> > be dev_t, and user land will never notice anything.
> 
> Glibc disagrees with you (bits/types.h):
> 
> typedef __u_quad_t __dev_t;             /* Type of device numbers.  */
> 
> We'd have to use __kernel_dev_t instead which again pulls kernel
> headers in..

Argg.  That's also non-funny:

[hch@sb linux]$ grep __kernel_dev_t; include/asm-*/posix_types.h
include/asm-alpha/posix_types.h:typedef unsigned int	__kernel_dev_t;
include/asm-arm/posix_types.h:typedef unsigned short	__kernel_dev_t;
include/asm-cris/posix_types.h:typedef unsigned short	__kernel_dev_t;
include/asm-i386/posix_types.h:typedef unsigned short	__kernel_dev_t;
include/asm-ia64/posix_types.h:typedef unsigned int	__kernel_dev_t;
include/asm-m68k/posix_types.h:typedef unsigned short	__kernel_dev_t;
include/asm-mips64/posix_types.h:typedef unsigned int	__kernel_dev_t;
include/asm-mips/posix_types.h:typedef unsigned int	__kernel_dev_t;
include/asm-parisc/posix_types.h:typedef unsigned int	__kernel_dev_t;
include/asm-ppc/posix_types.h:typedef unsigned int	__kernel_dev_t;
include/asm-s390/posix_types.h:typedef unsigned short	__kernel_dev_t;
include/asm-s390x/posix_types.h:typedef unsigned int	__kernel_dev_t;
include/asm-sh/posix_types.h:typedef unsigned short	__kernel_dev_t;
include/asm-sparc64/posix_types.h:typedef unsigned int	__kernel_dev_t;
include/asm-sparc/posix_types.h:typedef unsigned short	__kernel_dev_t;

Re: [linux-lvm] Re: [RFLART] kdev_t in ioctls

[Alan Cox]

> > Glibc disagrees with you (bits/types.h):
> > 
> > typedef __u_quad_t __dev_t;             /* Type of device numbers.  */
> > 
> > We'd have to use __kernel_dev_t instead which again pulls kernel
> > headers in..
> 
> Argg.  That's also non-funny:

glibc is meant to disagree. glibc provides a virtualised dev_t to user space
so that the kernel one can be expanded in future without application
breakage.

Re: [linux-lvm] Re: [RFLART] kdev_t in ioctls

[Christoph Hellwig]

On Mon, Jan 14, 2002 at 06:56:44PM +0000, Alan Cox wrote:
> > > Glibc disagrees with you (bits/types.h):
> > > 
> > > typedef __u_quad_t __dev_t;             /* Type of device numbers.  */
> > > 
> > > We'd have to use __kernel_dev_t instead which again pulls kernel
> > > headers in..
> > 
> > Argg.  That's also non-funny:
> 
> glibc is meant to disagree. glibc provides a virtualised dev_t to user space
> so that the kernel one can be expanded in future without application
> breakage.

I know - still it makes Linus' suggestion not work on ~90% of the
systems.

Re: [linux-lvm] Re: [RFLART] kdev_t in ioctls

[Linus Torvalds]

On Mon, 14 Jan 2002, Christoph Hellwig wrote:
>
> I know - still it makes Linus' suggestion not work on ~90% of the
> systems.

It doesn't matter if user-land compilation breaks. As long as old binaries
work (and they will), we're fine.

User-land was _already_ broken. By virtue of using a type that it should
NOT have used.

If you want to use __kernel_dev_t, go ahead.

		Linus

[linux-lvm] Re: [RFLART] kdev_t in ioctls

[Alexander Viro]

On Mon, 14 Jan 2002, Linus Torvalds wrote:

> The good news is that the bit-for-bit representation of old kdev_t and
> "dev_t" are obviously 100% the same, so we should just make the damn thing
> be dev_t, and user land will never notice anything.
> 
> So we can just change all structures that are exported to user land to use
> "dev_t", and add the required conversion magic. Possibly by duplicating
> the structure, and having "used_lvm_struct_x" and functions to read and
> write them from/to user space.

... that, BTW, would kill the crap like

typedef struct ...
	...
#ifdef __KERNEL__
        struct kiobuf *lv_iobuf;
        sector_t blocks[LVM_MAX_SECTORS];
        struct kiobuf *lv_COW_table_iobuf;
        struct rw_semaphore lv_lock;
        struct list_head *lv_snapshot_hash_table;
        uint32_t lv_snapshot_hash_table_size;
        uint32_t lv_snapshot_hash_mask;
        wait_queue_head_t lv_snapshot_wait;
        int     lv_snapshot_use_rate;
        struct vg_v3    *vg;

        uint lv_allocated_snapshot_le;
#else
        char dummy[200];
#endif
} lv_t;

Yup, structure "shared" by kernel and userland.  Look through the
include/linux/lvm.h - it's choke-full of that.  And quite a few
of them contain fields like
	struct proc_dir_entry *foo
	struct block_device *bar
	struct list_head baz
outside of these #ifdef __KERNEL__...
 
> > Public statement along the lines "any API that passes kdev_t values
> > across the kernel boundary is unacceptable" would be a nice thing...
> 
> Consider that done. ANYTHING that exports kdev_t to user space is
> incredibly broken, and will not work in a few months when the actual bit
> representation (and size) will change.
> 
> Do we have any lvm people willing to fix this? (linux-lvm cc'd, but I know
> they've been very silent on the 2.5.x changes so far)

Umm...  Is the version in main tree actually maintained?

Re: [linux-lvm] Re: [RFLART] kdev_t in ioctls

[Joe Thornber]

On Mon, Jan 14, 2002 at 10:01:25AM -0800, Linus Torvalds wrote:
> Consider that done. ANYTHING that exports kdev_t to user space is
> incredibly broken, and will not work in a few months when the actual bit
> representation (and size) will change.

The kdev_t's in the driver interface are just one of the *minor*
problems with the LVM driver.

I came to the conclusion last summer that a rewrite was in order, of
both the kernel driver and the userland tools.  The new driver is
called 'device-mapper', and has been discussed briefly on this list.
It aims to support volume management in general, ie. not be LVM
specific.

The userland tools (known as LVM2), will go into beta this week.
Initially they will just replicate the functionality of LVM1, but we
do have a lot of extra features queued which will go in subsequent
releases.

Of course Sistina will continue to support the existing LVM1 driver
for the 2.4 series.

As far as the 2.5 series is concerned, I would much rather see people
embracing the new architecture (or telling me why it sucks).  Rather
than trying to hack the LVM1 driver so it works.  People have been
complaining for the last year about LVM, we weren't able to do much
about it since we were in a stable kernel and couldn't change any
interfaces.  Now that 2.5 is finally here it is time for people to
address the real problems - kdev_t's only scratch the surface.

- Joe

Re: [linux-lvm] Re: [RFLART] kdev_t in ioctls

[Andries.Brouwer]

    On Mon, 14 Jan 2002, Christoph Hellwig wrote:
    >
    > I know - still it makes Linus' suggestion not work on ~90% of the
    > systems.

    It doesn't matter if user-land compilation breaks. As long as old binaries
    work (and they will), we're fine.

    User-land was _already_ broken. By virtue of using a type that it should
    NOT have used.

    If you want to use __kernel_dev_t, go ahead.

            Linus

Yes. As everyone knows, one should not use kernel includes.
On the other hand, having local copies of everything is also
a bad habit, to be avoided when possible.
With Linux it is generally impossible to avoid going to local
copies, but so far losetup survived with the construction

% cat loop.h
#include <linux/posix_types.h>
#undef dev_t
#define dev_t __kernel_dev_t
#include <linux/loop.h>
#undef dev_t
%

Yecch.

(This is terribly ugly, but the alternative may be even worse:
lots of #ifdef's testing architecture etc.)

It is a pity that dev_t, a type that is not used anywhere in the
kernel except at the interface with user space, is a different
type from what user space uses.

Andries